1. Field of the Invention
The present invention relates to a magnetic displacement detecting device coupled to a steering shaft of, for example, an automobile, to detect a steering angle of a steering wheel and, more particularly, to a magnetic displacement detecting device that permits highly accurate detection of a moving object that linearly moves.
2. Description of the Related Art
FIG. 9 is a top plan view showing a conventional magnetic displacement detecting device 40. The magnetic displacement detecting device 40 has a detection member 41, a fixed member 42, a magnet 43, and a Hall element 44 serving as a magnetic detecting means.
The detection member 41 is supported by a rod-shaped guiding member 45, and the detection member 41 linearly moves along the guiding member 45. The detection member 41 is provided with the magnet 43 having a hexahedron shape, which has been magnetized so that one end in a traveling direction of the detection member 41 carries the north pole, while the other end carries the south pole. The fixed member 42 is secured to a surface of a case (not shown) that opposes the magnet 43. The Hall element 44 is provided on surface of the fixed member 42 that opposes the magnet 43. The Hall element 44 detects an intensity of a component in a direction of z of a magnetic force emitted from the magnet 43.
For instance, in a magnetic displacement detecting device for detecting a steering angle of a steering wheel, a torque of a rotator that rotates together with the steering wheel is converted into a linear moving force of the detection member 41. The magnet 43 moves together with the detection member 41, opposing the Hall element 44, and a changing rate in the z-direction component of a magnetic field emitted from the magnet 43 at this time is detected through the Hall element 44, thereby allowing a rotational angle of the steering wheel to be known.
In this type of magnetic displacement detecting device 40, the Hall element 44 detects the intensity of a magnetic field in the z-direction, i.e., a direction orthogonal to a traveling direction of the detection member 41. Based on a changing rate in detection values of the magnetic field in the z-direction, a travel amount of the detection member 41 is recognized.
As shown in FIG. 9, however, the surface of the conventional magnet 43 that opposes the Hall element 44 is flat in the traveling direction. Therefore, a changing rate in the intensity of the magnetic field in the z-direction with respect to a traveling distance is relatively large in the vicinity of both ends in the traveling directions of the magnet 43. In contrast, at a central portion of the magnet 43 in the traveling direction, an area wherein a magnetic force line E0 is substantially flat in the traveling direction expands, and a changing rate in the intensity of the magnetic field in the z-direction with respect to the traveling distance of the magnet 43 becomes extremely small.
For the above reason, a changing rate of the intensity of the magnetic field in the z-direction with respect to the traveling distances of the detection member 41 and the magnet 43 becomes nonlinear, making it difficult to accurately grasp the traveling distance of the detection member 41.
To detect the steering angle of the steering wheel mentioned above, the detection member 41 is moved by the rotator that rotates together with the steering wheel. Another rotation detecting means is provided that detects the rotational angle of the rotator with a higher resolution than that of the Hall element 44. In a range wherein the Hall element 44 moves from the center of the magnet 43 to a front or rear end thereof, rough detection of xc2x1720 degrees of the rotator is performed. During the rough detection, more accurate rotational angle of the rotator is detected by the foregoing rotation detecting means. At this time, a further accurate angle is recognized by the rotation detecting means, by using a detection angle based on a detection value supplied from the Hall element 44.
In such a device, if the accuracy of the detection member 41 for detecting the moving distance deteriorates, causing a difference between the rotational angle of the rotator and a detection value, then a reference value used for performing detailed angle detection of the rotator varies. As a result, an error occurs in the detection of the rotational angle of the steering wheel.
Accordingly, the present invention has been made with a view toward solving the problem described above, and it is an object of the present invention to provide a magnetic displacement detecting device for detecting a relative travel amount as linearly as possible by a magnetic detecting means in a case where a detection member having a magnet and a magnetic detecting means relatively move.
To this end, according to one aspect of the present invention, there is provided a magnetic displacement detecting device having a detection member that linearly moves and a fixed member provided such that it opposes the detection member, one of the detection member or the fixed member being provided with a magnet having a north pole and a south pole in a traveling direction of the detection member, while the other being provided with a magnetic detecting means, and a magnetic force emitted from the magnet being detected by the magnetic detecting means thereby to detect a movement of the detection member, wherein a distance between the magnet and the magnetic detecting means is the smallest in the vicinity of a center of the magnet and increases toward ends of the magnet.
Shaping the magnet so that it bulges at its center toward the magnetic detecting means causes a changing rate of a magnetic field in a direction orthogonal to a traveling direction of the magnet to be linear or nearly linear over a longitudinal full length of the magnet in the traveling direction. With this arrangement, the relative travel distances of the detection member and the magnetic detecting device can be detected with higher accuracy by making use of a changing rate in the magnetic field.
In a preferred form of the present invention, a surface of the magnet that opposes the magnetic detecting means has a radius shape, including a convex surface in addition to a precise radius shape. denoted by H, and the radius of the curved surface is denoted by R, then the magnet preferably has a shape represented by: W=10 to 20, H=1 to 5, and R=20 to 50.
In another preferred form of the present invention, the surface of the magnet that opposes the magnetic detecting means is trapezoidal.
In this case, if a width of the magnet in a direction in which the magnet or the magnetic detecting means travels is denoted by W, a thickness of the magnet in a direction in which the magnet and the magnetic detecting means face each other is denoted by H, a length of cutoffs in the H-direction of tapers at both ends of the trapezoid is denoted by Y, and a length of cutoffs in the W-direction of the tapers is denoted by X, then the magnet preferably has a shape represented by: W=10 to 20, H=1 to 5, X=4.25 to 4.75, and Y=0.4 to 0.6.
In yet another preferred form of the present invention, the detection member is provided with a rotator, and the detection member linearly is operated by a rotational operation of the rotator, and further provided with an additional detecting means for detecting a rotational angle of the rotator with a higher resolution than that of a detection output obtained by the magnet and the magnetic detecting means together.
In this case, by using positional recognition based on a change in a magnetic field intensity detected by the magnetic detecting means as a reference, a detailed rotational angle of the rotator is determined from a detection output from the additional detecting means. Thus, highly accurate positional detection can be achieved by the. detection member and the magnetic detecting means. This prevents variations in a reference value for determining a more detailed rotational angle, permitting highly accurate detection of a rotational angle of the rotator.